1. Field of the Invention
This invention relates to means for damping vibration of structural members.
2. Description of the Prior Art
Seals are often employed within an engine to prevent fluid from leaking from one section of the engine to another. Such seals are commonly located between a stationary structural member and a moving structural member of the engine and can be carried by either member. When relative movement between structural members occurs, a form of wave motion, known as vibration, can develop in either structural member, but particularly in the one carrying the seal. The type and degree of vibration which develops depends upon such factors as engine speed and the design of the structural member itself.
In a turbine engine, seals are often carried on and can be integral with an annular rotating structural member, with the seal occasionally contacting or operating with small clearances with respect to an annular stationary structural member, called a stationary seal. The annular structural member is frequently of thin-walled construction, for weight and cost saving reasons, and therefore is very susceptible to vibration, such as circumferentially propagating flexural vibration and axially propagating vibration. Vibration of the structural member not only can result in lower engine efficiency, due to loss of effective sealing, but can also result in destructive fatigue or cracking of the seal or structural member. Means which reduce, or damp, such vibration are thus desirable in order to maintain engine efficiency and increase the useful life of the structural member.
One such means commonly used for damping vibration of an annular structural member is a damper ring. A damper ring is usually annular and concentric with the structural member and can be disposed either radially outward or radially inward of the structural member. It is normally held against the structural member in suitable manner.
For reasons to be explained hereafter, the damping effectiveness of a damper ring increases with an increase in distance between the neutral axis of the damper ring and the neutral axis of the structural member to be damped. The neutral axis of a body is the axis formed by the points of zero stress in the body. On one side of the neutral axis, the body is subject to tensile stress, while simultaneously on the other side of the neutral axis, the body is subject to compressive stress. Such stresses within the body become greater as the distance from the neutral axis increases. For example, at the interface between a radially inner surface of a structural member and a damper ring, during periods of vibration, the structural member will be in compression while the damper ring will be in tension. By increasing the distance between neutral axes, the compressive stresses of the structural member and the tensile stresses of the damper ring at the interface of the structural member and damper ring are also increased. Because of the resulting greater differential between the compressive stresses of the structural member and the tensile stresses of the damper ring, slip is promoted. Slip is the relative tangential movement between the damper ring and the structural member. When the structural member vibrates, the damper ring does not follow the vibration but rather slips or slides tangentially relative to the structural member. As a result of frictional effects, such relative movement produces heat and thereby promotes energy dissipation. That is, slip causes the energy of vibration to be dissipated in the form of heat. Thus, by increasing the distance between the neutral axis of the damper ring and the neutral axis of the structural member, slip, and thereby damping effectiveness, is increased.
Most currently used damper rings are located adjacent or almost adjacent the structural member which is to be damped. For example, some structural members include a groove, or ring trap, within themselves into which the damper ring fits. This results in the neutral axes of the damper ring and the structural member being relatively close to each other. Damping effectiveness is thereby decreased. Furthermore, thermal sinks, vibration stiffeners, or other devices are often added to the structural member. These devices are normally radially extending flanges of the structural member itself. When the damper ring is disposed on the same side of the structural member as are the aforementioned devices, the neutral axes of the damper ring and the structural member are brought even closer together, with a corresponding decrease in damping effectiveness.
Another problem which is encountered with damper rings is retaining them in a proper location and position relative to the structural member, while at the same time permitting ease of installation and replacement. Both axial movement and radial movement of the damper ring must be prevented, while, as previously indicated, relative tangential movement, or slip, between the damper ring and the structural member must be permitted. Although prior art damper rings have been constructed of metal and severed axially to meet some of the above requirements, none are known to have met all of those requirements as successfully as does the present invention.
In view of the above-mentioned problems, it is therefore a primary object of the present invention to provide new and improved means for effectively damping vibration of a structural member.
Another object of the present invention is to provide a damper ring which promotes slip between it and the structural member.
Still another object of the present invention is to provide a damper ring which can be retained in its proper location and position and yet which is easy to install and replace.